1. Field of the invention
The present invention relates to a sewing machine driving system, and more particularly to a sewing machine driving system including a controller for operating a sewing machine at a desired speed to sew a fabric piece and thereafter stopping a sewing needle at a prescribed position.
2. Description of the Prior Art
Various control systems for sewing machine drivers having a needle position stopping capability are known in the art. For example, U.S. Pat. No. 3,910,211 discloses a control system employing an electromagnetic clutch and brake system. The control system shown in U.S. Pat. No. 4,080,914 comprises an eddy-current braking system. According to U.S. Pat. No. 4,137,860, a DC motor control system is disclosed for a sewing machine.
The electromagnetic clutch and brake system includes a clutch motor having a coupling which comprises a combination of an electromagnetic clutch and an electro-magnetic brake for changing the speeds of rotation of the motor and stopping the motor.
As shown in FIG. 8 of the accompanying drawings, the disclosed coupling includes a flywheel 2 fixed to the output shaft 1 of an induction motor, the flywheel 2 being rotated at all times while the the motor is being energized. When there is no load on the motor, the flywheel 2 stores rotational energy. A friction disc 3 is mounted on an outer side of the flywheel 2, and another friction disc 5 is mounted on a bracket 4 which is positioned in confronting relation to the flywheel 2. Between the friction discs 3, 5, there are disposed a movable clutch disc 8 and a movable brake disc 9 which are axially slidable on a spline sleeve 7 forced-fitted over an output shaft 6. Linings 10, 11 are fixed respectively to the outer sides of the clutch and brake discs 8, 9 which face the friction discs 3, 5, respectively. The discs 8, 9 have outer peripheral surfaces providing a portion of a magnetic path formed by electromagnets 14, 15 that are energized by respective coils 12, 13.
The coupling thus constructed operates as follows: When the electromagnet 14 is energized, a magnetic flux flows through the friction disc 3 and the outer peripheral edge of the movable clutch disc 8 to magnetically attract the movable disc 8 toward the flywheel 2. As the disc 8 is thus moved axially, the lining 10 is pressed against the friction disc 3 as it rotates, whereupon the torque of the flywheel 2 is transmitted through the spline sleeve 7 to the output shaft 6.
Upon enegization of the electromagnet 15 under this condition, a magnetic flux flows through the outer peripheral edge of the movable brake disc 9 and the friction disc 5 to magnetically attract the disc 9 toward the bracket 4. This axial movement of the disc 9 presses the lining 11 against the friction disc 5 to couple the output shaft 6 to the bracket 4, thus braking the output shaft 6.
The currents flowing through the coils 12, 13 may be controlled to provide a partly connected clutch condition.
The output shaft 6 is operatively connected by a belt and pulleys to a sewing machine drive shaft. The motor is controlled in speed by a signal fed back from a speed sensor mounted on the sewing machine drive shaft.
Sewing machines for industrial use with a needle position stopping capability and a thread cutting capability are required to provide an intermediate operation speed. To obtain such an intermediate operation speed, the coupling is controlled at the partly connected clutch condition, in which the linings 10, 11 are worn of necessity. If wrong materials were selected for the linings 10, 11, the linings 10, 11 would be responsible for troubles.
The disclosed coupling requires constant maintenance since the worn linings 10, 11 must be replaced. However, the servicing of the linings 10, 11 is problematic because they're not worn uniformly.
The eddy-current braking system employs an eddy-current coupling in place of the coupling of the electromagnetic clutch and brake system. The eddy-current coupling is better than the electromagnetic clutch and brake system in that there is no lining wear problem inasmuch as the torque output is transmitted without any physical contact.
The eddy-current coupling mechanism is shown in FIG. 9 of the accompanying drawings. An induction motor has a motor shaft 20 with a rotating member 21 mounted thereon. The rotating member 21 comprises a driver 21a made of a nonmagnetic material, a claw pole 21b connected to the driver 21a, a nonmagnetic member 21c mounted on a distal end of the claw pole 21b, and a yoke 21d joined to the nonmagnetic member 21c.
A cup-shaped cylindrical member 24 of copper is mounted by a hub 23 on an output shaft 22 and extends into a gap defined between the claw pole 21b and the yoke 21d. The induction motor also has an intermediate bracket 25 to which an excitation coil 27 is attached by a ring-shaped steel plate 26. When the excitation coil 27 is energized, a magnetic flux is generated as indicated by the broken lines.
When the magnetic flux is generated by energization of the excitation coil 27, it flows from the claw pole 21b through the cylindrical member 24 as the rotating member 21 rotates. This magnetic flux is equivalent to a rotating magnetic field applied to the cylindrical member 24, causing an eddy current to be produced in the cylindrical member 24.
The eddy current and the claw pole 21b coact to produce an attractive force between the cylindrical member 24 and the claw pole 21b for transmitting the motor torque from the motor shaft 20 to the output shaft 22 without any physical contact. Since the transmitted torque varies by changing the magnitude of the exciting current flowing through the excitation coil 27, the speed of rotation of a load coupled to the output shaft 22 can be controlled in a stepless manner by changing the magnitude of the exciting current.
Problems with the eddy-current braking system are that since the cylindrical member 24 is of a coreless structure for desired response, the thermal capacity thereof is limited and the permeance thereof is low thus limiting the magnitude of the magnetic flux. As a result, the transmitted torque is low.
With the eddy-current braking system as well as the electromagnetic clutch and brake system, the motor has to be rotated at all times, and hence the power consumption of the motor while the coupling is not in operation and the noise of the motor while it is idly rotating are disadvantageous.
The DC motor control system employs a DC servomotor. The DC motor control system eliminates the problems of the electromagnetic clutch and brake system and the eddy-current braking system, and can perform ideal sewing machine control because of its high response. The motor is normally de-energized since it is started by depressing a sewing machine pedal. Accordingly, a large amount of electric power can be saved and there is no noise problem.
However, the DC motor suffers from the problem of brush wear. Where the sewing machine is used very often and transformerless AC-to-DC conversion is effected in a high-voltage region (such as in Europe), some measure must be taken to reduce brush wear. When the brush service life is terminated, the brush must be replaced and brush powder must be removed. Therefore, the motor requires maintenance relatively frequently.
The applicant has found that all of the above conventional drawbacks can be removed by designing a DC motor control system with a brushless motor, and directed attention to an AC servomotor system taking advantage of semiconductor control technology which has been advanced rapidly in recent years. The applicant has considered a system in which a synchronous motor with a permanent magnet field is used and a system in which an induction motor is used. These motors require a power converter composed of a converter and an inverter. It has been found that since the motors can be driven at variable speeds primarily by controlling the inverter, the same speed control as that of the DC motor can be achieved even though the motors are brushless.
Although the synchronous motor with a permanent magnet field only needs a relatively simple control circuit, the permanent magnet is disposed on a rotor side and there is a certain problem as to how the permanent magnet is fixed. In addition, the permanent magnet tends to be demagnetized by an overcurrent and a peak current of the stator. The synchronous motor with a permanent magnet field is expensive to construct because a high-resolution encoder or a costly resolver must be used in order to accurately detect pole positions.
The induction motor is rugged and inexpensive inasmuch as the rotor comprises an aluminum die casting rotor. However, a loss on the stator is large because the stator is relied upon for the supply of electromagnetic energy, and the temperature rise due to a copper loss on the rotor which arises from the generation of a secondary current is higher than that of the synchronous motor. The controller for the induction motor is rendered complex and expensive by the use of a transvector system and means for compensating for a change in the secondary resistance.